Graphene is becoming an ever popular material to materials scientists for use in a wide range of applications. Its structure gives rise to some interesting electronic and mechanical properties that have propelled it to the forefront of its field.
Graphene is a monolayer or single sheet of graphite and is the newest member of the nanocarbon family, which includes carbon nanotubes and fullerenes. The two-dimensional single-layered continuous network of hexagonally arranged carbon atoms gives rise to some interesting electronic properties; in particular the relativistic behaviour of the conduction electrons, termed Dirac fermions, which travel at speeds only 300 times less than the speed of light.
Graphene has also been shown to exhibit a room temperature quantum Hall effect and an ambipolar field effect, with high carrier mobility (approaching 200,000 cm2V−1s−1), where the charge carriers can be tuned between electrons and holes. Aside from the obvious interest in the unique electronic properties, graphene has exceptional strength with mechanical properties rivalling that of carbon nanotubes with a Young's modulus of 1 TPa.
The thermal conductivity of graphene is also comparable to carbon nanotubes with values up to 5300 Wm−1K−1 recorded. Owing to these exceptional properties graphene has applications in new generation electronic components energy-storage materials such as capacitors and batteries, polymer nanocomposites, optically transparent thin films, printable inks and mechanical resonators.
Problems arise due to the scalability of most existing graphene synthesis methods.
Methods exist to produce high quality continuous films of material that are suitable for some electronics applications. However, in fields such as composite science; the fabrication of large area films for transparent electrode applications; capacitors; and inks, much larger quantities are required.
Graphene has been made by a number of methods including micromechanical cleavage, sublimation of silicon from SiC in ultra high vacuum, chemical vapour deposition (CVD) growth on metal foils, liquid exfoliation of graphite and graphite oxidation to graphene oxide followed by reduction.
The problem with making graphene is that existing methods are useful for single flake studies and for relatively large area continuous films, but methods for preparing it in scalable quantities is very difficult. Samples prepared by graphite oxidation and reduction have become available—but are difficult to scale and the reduction step only partially restores the pristine graphitic structure, so the resulting material does not possess the excellent mechanical and electrical properties associated with graphene.
The growth mechanism for CNTs involves the catalytic decomposition of the carbon precursor molecules on the surface of the metal catalyst particles, followed by diffusion of the released carbon atoms into the metal particles. Carbon saturation in the metal occurs by reaching the carbon solubility limit whereby a carbon envelope is created and grows with continued carbon precipitation forming tube structures.
CNTs have been synthesised by growing graphitic carbon around nickel nanowire templates, and spherical nanoparticles have been used to template the growth of carbon nanotubes using catalytic vapour deposition.
Few layer graphene nanoribbons (4.5-7.5 nm thickness) have also been grown by CVD of methane/hydrogen mixtures on ZnS nanoribbons on silicon substrates. Wei et al J. Am. Chem. Soc. 2009, 131, 11147 describes a method of producing graphene ribbons for use in electronics applications, using zinc sulphide ribbons on silicon substrates as templates for graphene growth, produced by chemical vapour deposition.
In this method, the silicon substrate is needed to grow the zinc sulphide ribbons in situ. This makes the method very difficult to scale as it is limited by the area of the silicon substrate. Also, the method requires a step of mechanically removing graphene from the substrate, by scratching it from the silicon.
It is known to grow large area graphene on copper foil sheets to produce a continuous layer of graphene in the form of a large sheet for use in electronic devices. However, this process is very expensive and is therefore undesirable for large scale production.
In methods utilising metal foils, the amount of graphene produced is similarly limited by the area of the starting substrate/foil.
Additionally, chemical vapour deposition on foils has limited application where a continuous film is required.
It would be desirable to provide an improved process for producing graphene.